Leucine and isoleucine are amino acids which differ in the arrangement of the CH groups in the neutral residue.
Further properties may be found in the following table:
______________________________________ L-leucine L-isoleucine ______________________________________ Molecular weight [g/mol] 131.17 131.17 Iso-electric point pH 5.98 pH 5.94 Solubility [g/100 ml] 2.19 4.12 ______________________________________
Mixtures containing both of these amino acids are found, for example, in protein hydrolyzates. As is known from the prior art, there are considerable difficulties in separating L-isoleucine from other branched amino acids, such as L-leucine or L-valine (Ullmann, vol. A2, p. 70, 5th edition, 1985).
Proposed solutions have included, for example, fractionated crystallization of hydrochlorides (Japanese patent JP 39-1915 (1964)) or conversion of these amino acids into their cobalt salt and selective extraction of the corresponding L-isoleucine salt with alcohols from the solid phase (Japanese patent JP 37-15118 (1962), cited in H. Samejima The Microbial Production of amino acids, Tokyo 1972, p. 245 & p. 253).
While it is indeed known from German patent application P 42 17203.9 (European patent application 93 105321.9) that it is possible to isolate amino acids from aqueous solutions using zeolites, there is no indication of the separation of the closely related amino acids L-leucine and L-isoleucine from each other.